The main function of thermal insulation is to decrease the transfer of heat into or from an insulated system, whichever the case may be, in order to protect a device, such as an appliance, vessel, pipeline or other apparatus, from the effects of a gain or a loss of heat from or to the outside environment.
There are increasing concerns about the energy levels consumed by residential appliances, such as freezers, refrigerators and hot water heaters, which have created a need for an efficient insulation capable of meeting a demanding combination of technical, economic and environmental requirements. High thermal resistance e.g. R in excess of 8 hr ft.sup.2 .degree.F./inch BTU, low density e.g. under 10 lbs/ft.sup.3, adequate structural strength, load-bearing capability e.g. around 2000 lbs/ft.sup.2, flexibility, ease of application, low cost and a low aging factor are all important properties. Thermal resistance R is measured by the procedure of ASTM C1114-89.
Heat may be transferred from a heat source to the heat receiver by one or more of the heat transfer modes viz. conduction, convection and radiation. Conduction involves heat transfer due to interaction of atoms or molecules possessing a greater amount of kinetic energy with those possessing less. When the molecules are fixed in space, as in solid bodies, interaction of molecules responsible for thermal conduction arises from the elastic binding forces between the molecules. When the molecules are not fixed in space, as in gases or liquids, heat conduction is produced by the transfer of kinetic energy during molecular collisions.
Convection involves heat transfer by the actual movement of a fluid. When the fluid is free to move as in gases or liquids, portions of the fluid in contact with the heat source become hotter, expand, become less dense and rise. Their place is taken by the denser and colder portions of the fluid. This process generates natural convection currents that in gases at ordinary pressures are responsible for the major proportion of the heat that is transferred. The contribution of convection to the overall heat transfer in a gas can be reduced or eliminated by lowering the gas pressure. Lowering the pressure of a gas contained within a vessel reduces the number of molecules of the gas per unit volume. When the pressure level is reached at which the distance between the walls of the vessel is much smaller than the length of the mean free path of the gas molecules at the given conditions, the convection contribution is effectively eliminated.
Radiation involves the transfer of radiant energy from a source to the receiver. A solid body at any temperature above absolute zero radiates energy. This radiation is electromagnetic in nature and takes place without the necessity of an intervening medium. A part of the radiant energy impacting a receiver is absorbed and a part is reflected by it. The contribution by radiation to overall heat conduction can be reduced by interposing radiation shields between the heat source and the receiver.
The contribution of each of the above modes to the overall heat transfer depends on the heat transfer medium as well as on the temperature and temperature differential between the heat source and the receiver. Under certain conditions, any one of the three modes may become controlling, while, under other conditions, the contribution of two or of all three modes of heat transfer may be significant. Combinations of component materials may be used to emphasize certain more desirable properties in an article and to suppress other less undesirable ones. The resultant composite multicomponent thermal insulations can have better overall structural characteristics and thermal insulating properties than those of the individual components thereof.
Thermal insulation is intended to reduce the contribution of all the modes of heat transfer to a practical minimum. It is usually made of poor heat conductors and may be comprised of one or two phases, usually solid and/or gas. Examples of thermal insulation include solid insulation panels made of low conductive materials, expanded foams, gas-filled or evacuated powders and fibrous materials, vacuum alone, opacified powders and multilayer insulations. Each of these types of insulation has its advantages, disadvantages and limitations. Thermal resistance, structural integrity and load-supporting capability are of paramount importance. The overall effectiveness of an insulation is considerably reduced when the insulation is not capable of supporting loads and when solid supports are required. The selection of a specific type of insulation for a particular type of service is made on the basis of a compromise between factors such as effectiveness, cost, ruggedness, compatibility, applicable temperature range, aging factor and ease of application.
Gas-filled, closed-cell plastic foams e.g. polystyrene, polyurethane, polyisocyanurate, are among the most economic and efficient existing types of insulation for medium temperature range applications. They have been extensively used for insulating freezers and refrigerators. Foam-type insulation has a cellular structure generated by the expansion of a foamable composition, often referred to as a foamable resin, plastic or polymer composition. It contains two phases, viz. a gas phase and a solid phase. The conductivity of foam insulation is determined by the sum of the heat flow through the gas contained within the cells and through the network of the plastic cell walls. The heat flow through the closed-cell foam insulation can be reduced by filling the cells with a low conductivity gas, by extending the length of the heat flow path through the solid phase, and by reducing the thickness of the cell walls.
The effectiveness of insulations is measured in terms of thermal resistance (a reciprocal of thermal conductivity) expressed as R/inch values (hr ft.sup.2 .degree.F./BTU inch), and often referred to as the R value. The highest practical R values of closed-cell foams are about 7-8, attainable by CFC-blown polyurethane or polyisocyanurate foams. The effective R values of closed-cell foam insulation tend to decrease with age due to a gradual replacement of the cell gas with more conductive air by the process of diffusion. Other disadvantages of closed-cell foams are relatively high coefficient of thermal expansion resulting in a tendency for closely-fitted insulation to crack during the temperature cycle of the appliance. Cavities are often found inside the foamed-in-place insulations. Moreover, current environmental regulations prohibit the use of ozone-destroying chlorofluorocarbons (CFCs). All practical substitutes have higher thermal conductivity and the replacement of CFCs with such substitutes results in a decrease of the thermal resistance of this type of insulation.
Thermal resistance of plastic foams could be enhanced by lowering the pressure of the gas contained within the foam cells to a very low level. This requires an open cell foam, located inside an impermeable casing which has been evacuated. Examples of this type of foam are given in U.S. Pat. Nos. 4,647,498 of Walles and 4,668,555 of Uekado et al. The R values attainable by such insulation are determined by the heat conduction through the cell walls, the conductivity of the interstitial gas and the residual convective heat flow. The disadvantages tend to be residual heat leak through the cell walls, high cost, poor load-supporting capability, and structural integrity limitations.
Gas-filled fibres or powders allow reduction of the heat flow by the modes of conduction, convection and radiation to a level unattainable with foam-type insulation. In the case of foams, the conductive heat path through the cell walls is continuous. In the case of powders or fibres, the conductive heat flow path from particle to particle is limited to the points of contact between the particles and is impeded by the phase discontinuities. The contribution of convective heat flow can be made very low or be entirely eliminated by reducing the interstitial gas pressure and/or reducing the size of the particles so that the equivalent diameter of the voids is equal to or smaller than the mean free path of the gas molecules at the given temperature and pressure. The contribution of radiation is reduced to a very low level as each particle acts as a radiation shield. Examples of nonevacuated and evacuated types of particle insulations are given in U.S. Pat. Nos. 3,695,483 to Pogorski and 4,681,788 to Barito et al.
Insulations made of compacted fibres or powders enclosed in gas-impermeable evacuated casings can attain R/inch values well in excess of 8. The main disadvantage of this type of insulation is cost and weight. The finer and the more compacted are the particles, the higher is the thermal resistance of the insulation, the load-supporting capability and the structural integrity of the insulation, but also the density is higher, the evacuation time required is longer and the cost is higher. Conversely, the lesser the compaction, the lower is the thermal resistance and the poorer is the structural integrity and load-supporting capability.
The foregoing discussion of the prior art indicates that previous types of insulation for moderate temperature service tend to have one or more undesirable characteristics, e.g., low thermal resistance, poor load-supporting capability, poor structural integrity, high density and/or high cost. Any of these factors can be serious enough to considerably affect or limit the usefulness of such types of insulation.